-Wconversion misses non-elided integral promotion.

[Quuxplusone]

Bugzilla Link	PR37170
Status	NEW
Importance	P normal
Reported by	Nicholas Baldwin (nicholasjbbaldwin@gmail.com)
Reported on	2018-04-18 14:43:33 -0700
Last modified on	2018-04-18 14:49:00 -0700
Version	trunk
Hardware	PC Linux
CC	llvm-bugs@lists.llvm.org
Fixed by commit(s)
Attachments	wconversion_bug.png (364801 bytes, image/png)
Blocks
Blocked by
See also

Integral promotion of smaller int types (e.g. uint8_t) can be elided when no
difference in observable behavior happens. Thus when -Wconversion is turned on
and the promotion is elided, no warning is reported as it should.

However, in the case where elision cannot happen (such as the following code
sample) and integral promotion clearly takes place, -Wconversion still reports
no error.

#include <stdint.h>
uint8_t test(void);

uint8_t test() {
    uint8_t a = 0x5a;
    uint8_t c = (uint8_t)(~(int)a) >> 4;
    return c;
}

will return 0x05 and produces no errors (correct).

#include <stdint.h>
uint8_t test(void);

uint8_t test() {
    uint8_t a = 0x5a;
    uint8_t c = (~a) >> 4;
    return c;
}

will return 0xf5 and still produces no errors (incorrect). Since a is clearly
being promoted to an int, -Wconversion should warn that a the integer
conversion is losing precision.

I've attached a screenshot of the difference in code gen.

[Quuxplusone]

Attached wconversion_bug.png (364801 bytes, image/png): Screenshot of godbolt excercising the issue.

[Quuxplusone]

(In reply to Nicholas Baldwin from comment #0)
> Integral promotion of smaller int types (e.g. uint8_t) can be elided when no
> difference in observable behavior happens. Thus when -Wconversion is turned
> on and the promotion is elided, no warning is reported as it should.
>
> However, in the case where elision cannot happen (such as the following code
> sample) and integral promotion clearly takes place, -Wconversion still
> reports no warnings.
>
> #include <stdint.h>
> uint8_t test(void);
>
> uint8_t test() {
>     uint8_t a = 0x5a;
>     uint8_t c = (uint8_t)(~(int)a) >> 4;
>     return c;
> }
>
> will return 0x05 and produces no warnings (correct).
>
> #include <stdint.h>
> uint8_t test(void);
>
> uint8_t test() {
>     uint8_t a = 0x5a;
>     uint8_t c = (~a) >> 4;
>     return c;
> }
>
> will return 0xf5 and still produces no warnings(incorrect). Since a is
> clearly being promoted to an int, -Wconversion should warn that a the
> integer conversion is losing precision.
>
> I've attached a screenshot of the difference in code gen.

Used incorrect noun, meant to say that it produces no warnings, not no errors.